Rendering can take up large amounts of memory. It depends on what you're rendering, but it's probably best to get more than the standard 8 GB. 16 GB is pretty affordable still, so that's my initial recommendation. If you get it as 2 x 8GB you can always upgrade with another 2 x 8GB later if necessary.
For gaming and regular use, 8 GB would be plenty, so it's the rendering that dictates how much you'll need.
PS: If you haven't bought the GTX 660 Ti yet, you might want to consider getting a 760 instead.